1. Field of the Invention
The present invention relates generally to oxygen storage and retrieval systems and more particularly to such a system which provides an oxygen source which can selectively yield a controllable steady flow of oxygen.
2. Description of the Prior Art
Oxygen is the most abundant element on earth, making up approximately 50% of the planet's mass. Correspondingly, it plays an important role in many biological processes; for example, humans must consume several pounds of oxygen per day. Commercial and medical uses of oxygen are widespread, requiring approximately 20 millions tons of O.sub.2 annually.
Pure oxygen is produced primarily through air separation and is stored and shipped as a liquid or compressed gas. Liquid oxygen (LOX) has a critical point temperature of 154.58.degree. K. (-181.degree. F.). Thus, cryogenic O.sub.2 storage entails bulky insulated tanks and necessitates continuous venting of gas to the atmosphere. Pressurized oxygen gas requires massive cylinders, and since high pressure oxygen can react violently with organic materials, specifically cleaned fittings are required.
Oxygen candles, frequently used for emergency life support, have been a valuable source of O.sub.2. Their development can be traced back to the early 1940's when the need for a convenient source of oxygen for emergency use in aircraft and submarines became apparent. Cryogenic and high pressure O.sub.2 storage systems were poorly suited for this application because of the inherent bulk, weight, and complexity of the required containment vessels and delivery systems. One solution, developed and refined over the next several decades, was the "oxygen candle". This apparatus uses a pyrotechnic grain which, once ignited, reacts at a specified rate to yield a relatively large quantity of oxygen. A typical candle formulation is given below in Table 1.
TABLE 1 ______________________________________ TYPICAL OXYGEN CANDLE COMPOSITION Chemical Species Composition Purpose ______________________________________ NaClO.sub.3 80% Oxygen source Fe 10% Fuel Fiberglass 6% Binder BaO.sub.2 4% Chlorine scavenger ______________________________________
In this type of oxygen candle, iron reacts with a portion of the sodium chlorate to form mixed iron oxides. The heat generated by this highly exothermic reaction melts the remainder of the NaClO.sub.3, which decomposes to yield O.sub.2 and sodium chloride. Emergency breathing systems based on this type of oxygen candle are in service on most commercial airliners. They have been used as oxygen suppliers for evacuations from underground mine accidents, burning surface ships, and in submarine emergencies.
Typical of the prior art of oxygen candles are the U.S. Pat. Nos. 5,049,306 to Greer, 4,905,688 to Vicenzi et al., 4,891,189 to Harwood, Jr., 3,871,281 to Leonard et al., 3,615,251 to Klenk, 3,615,250 to Vernon, and 3,542,522 to Mausteller. Unfortunately, as in other systems which incorporate pyrotechnic grains (solid rocket motors, for example), there is no convenient way to modify the burn rate or stop the reaction once initiated. Other U.S. Pat. Nos. 4,642,142 to Hyyppa and 4,632,714 to Abegg et al. are concerned with pyrotechnic formulations which may contain some LiClO.sub.4 as an oxidizer although none of the compositions mentioned in these patents are intended to serve as sources of pure oxygen.
Each of the oxygen storage options described above has inherent limitations. There is a requirement for a convenient, controllable oxygen source which can be used in applications where cryogenic or high pressure storage are impractical.
A system has been proposed in which liquid LiClO.sub.4 would serve as a chemical oxygen source via the reaction: EQU LiClO.sub.4 .fwdarw.LiCl+20.sub.2 [ 1]
LiClO.sub.4 was chosen for this application for several reasons. First, as shown in Table 2, this species has the highest oxygen storage density of any chlorate or perchlorate.
TABLE 2 __________________________________________________________________________ PROPERTIES OF VARIOUS CHEMICAL OXYGEN SOURCES Grams O.sub.2 gas Mass needed to Volume needed to Density per cm.sup.3 produce 1 man-day produce 1 man-day Material (gm/cm.sup.3) Mass % O.sub.2 material O.sub.2 (grams) O.sub.2 (cm.sup.3) __________________________________________________________________________ LiClO.sub.4 2.43 60.1 1.45 1520 626 NaClO.sub.4 2.53 52.0 1.31 1742 689 KClO.sub.4 2.52 46.2 1.16 1964 777 LiClO.sub.3 1.12 53.0 0.59 1710 1526 NaClO.sub.3 2.49 45.1 1.13 2009 808 KClO.sub.3 2.32 39.2 0.91 2313 998 98% H.sub.2 O.sub.2 1.43 46.1 0.66 2009 1373 LOX 1.14 100. 1.14 907 797 __________________________________________________________________________
FIG. 2 shows the volumetric oxygen density of several storage media, including liquid LiClO.sub.4. Note that a given amount of the molten salt actually contains more oxygen than the same volume of liquid O.sub.2.
A second factor which makes this reaction attractive for oxygen generation applications is the fact that LiClO.sub.4 is the only perchlorate which is stable at temperatures significantly higher than its melting point, allowing this material to be conveniently handled as a liquid for casting into reaction vessels.
Batch Reactor
Reaction [1] exhibits temperature dependent kinetics. The decomposition of LiClO.sub.4 can be characterized as follows;
______________________________________ TEMPERATURE 590.degree. F. 774.degree. F. 945.degree. F. REACTION RATE SLIGHT RAPID VIOLENT ______________________________________
Because of this strong temperature dependence, it was initially assumed that the rate of oxygen production could be controlled by heating and cooling the perchlorate bath.
Although energy is needed to melt the perchlorate and initiate oxygen production, reaction [1] is exothermic. For this reason, the LiClO.sub.4 bath requires continual cooling to yield a constant oxygen flowrate. A test apparatus known as "batch reactor", utilized for this purpose maintained the bath temperature by spraying cold water on the outer surface of the reactor. A number of experiments were carried out using this apparatus with disappointing results.
In a typical experiment, the reaction vessel was filled with LiClO.sub.4 powder and the cartridge heaters energized to melt the perchlorate. This heating process took approximately 60 minutes. As the temperature of the bath rose, oxygen generation would commence. With constant bath temperature, a relatively steady flow of oxygen could be maintained for 60 to 90 minutes. Eventually, however, an increase in oxygen evolution would become apparent, as evidenced by an increased gas flow, accompanied by a bubbling sound from within the reaction vessel. Several minutes after the onset of increased O.sub.2 production, a brief but very intense reaction would take place. Violent gurgling noises from the reaction vessel would give way to a sudden and dramatic flow of gas, usually accompanied by the ejection of significant quantities of LiCl from the exhaust tube. The bath temperature would surge to temperatures on the order of 1200.degree. F. Indeed, bath temperature would increase from 900.degree. F. to 1175.degree. F. in less than 30 seconds. This process was generally accompanied by a rapid rise in reaction vessel pressure.
At this point, the reaction stopped altogether, with no further evidence of oxygen production. While the pressure spike accompanying this reaction was too brief to register on the data acquisition system (which had been set to collect 10 samples per second), it was clear that a very large portion of the total oxygen production took place in this short period of intense gas generation. The most likely explanation for the sudden increase in reaction rate (manifest in every test of the batch oxygen generation apparatus) is that the autocatalytic nature of reaction [1] dominated this final portion of the oxygen production process. The decomposition of LiClO.sub.4 is catalyzed by the LiCl reaction product. For a given temperature, the maximum rate of reaction [1] occurs when the LiClO.sub.4 reactant becomes saturated with the LiCl product. In addition to the autocatalytic effect noted above, the exothermic nature of the reaction appears to have played a significant role in the rapid surge of oxygen production associated with batch tests. The decomposition of LiClO.sub.4 took place so rapidly that the spray coolant system was unable to remove the heat generated by this process. The resulting increase in temperature also contributed to the runaway nature of the reaction.
Thus, it became clear that the chemical kinetic factors caused the batch oxygen generation system to be impractical.
In U.S. Pat. No. 3,709,203 to Cettin et al., a system is disclosed in which O.sub.2 produced from LiClO.sub.4, is used in place of atmospheric oxygen in an internal combustion engine. Although the gas generator portion of the Cettin et al. system is intended to serve as a controllable source of pure oxygen, it uses hot engine exhaust to raise the temperature of the perchlorate canister to the point at which a desired rate of O.sub.2 generation is achieved. In the operation of the Cettin et al. system, the thermal energy and the lithium chloride products remain in the perchlorate canister. Increased temperatures and higher concentrations of LiCl both have the effect of increasing the reaction rate, which in turn results in the production of more thermal energy and higher LiCl levels. These factors eventually combine to cause a sudden acceleration in oxygen production, which undesirably requires very rapid removal of heat to effect control in a thermally regulated system. Following the sudden acceleration of oxygen production, the system falls silent.
This is simply another variation of the batch oxygen generation system which, as described above, has been tried with disappointing results.